Effect of Magnet Strength on MRI Imaging of Distribution Following Local Delivery – A Pilot Study in Spatial Resolution.

Title: Effect of Magnet Strength on MRI Imaging of Distribution Following Local Delivery – A Pilot Study in Spatial Resolution.

Background: Drug distribution following local delivery has been measured using quantitative MRI at 7T. Documenting antimicrobial levels in post debridement sites clinically requires use of available resources with larger bore machine. Imaging sequences and characteristics do not scale directly from 7T technology to 3T to 1.5T clinical technology. This study compares resolution and image characteristics of locally delivered Gd-DTPA in a human cadaveric specimen between MRI field strengths.

Methods: Gadolinium loaded bone cement (GLBC) was formulated with 10gm Gd-DTPA per 40g batch Simplex P. A cadaveric humeral specimen was prepared by sub-periosteal exposure of the mid diaphyseal region and creating a longitudinal 4 cm x 1 cm cortical window. A 6 cm intramedullary rod and four 1.5cm x 10cm x 3mm sheets were fabricated from GLBC when it was in the dough phase. The rod was placed in the intramedullary canal distally and the sheets were molded around the periosteal surface and allowed to harden. The soft tissue was then closed with suture. On days 1,7 and 30 post implantation, distribution was imaged on 7T MRI. On the same days the specimen was imaged in a 3T MRI using a head coil and a 1.5 T MRI using an extremity coil and clinical sequences to produce fat-suppressed T1-weighted images.

Results: Locally delivered Gd-DTPA is visible penetrating tissue adjacent to the local delivery depot under all three field strengths. The T2* effect, which causes loss of signal due to the transition of contrast agents from paramagnetic behavior to ferromagnetic behavior, was found to be worst in the 7T unit, and greatly reduced, progressively in the 3T and 1.5T units. Encoding direction was also found to have a significant effect on artifact presence and amount of distortion.

Discussion: A major limitation of the current research on the 7T preclinical machine has been the emergence of T2* effects at concentrations expected in clinical practice. This pilot data is consistent with less T2* effect with the use of lower field strength clinical magnets.